1. Technical Field of the Invention
The invention relates to halftone reproduction of continuous tone images, and specifically to black and white radiographic images.
2. Background Art
Primary radiographic images are conventionally created by exposing a black and white photographic film or plate to an X-ray source and an interposed sample. The more absorbent parts of the sample throw shadows onto the photographic film or plate, and they appear less dark when the film or plate is developed. A radiograph of the human body, for example, shows the bones whiter than surrounding flesh because bones contain the element calcium, which has a relatively high atomic number. Abnormalities and foreign bodies are readily visible, and appropriate therapeutic action can be taken. Internal organs generally absorb X-rays to about the same extent as the surrounding flesh, but they can be shown up on a radiograph by concentrating material of greater absorbing power into the organ.
Radiography also has important industrial uses in locating internal defects in materials in creating three-dimensional images through stereoscopy, and in studying the three-dimensional structure of solids through tomography.
In all these applications, the exposed photographic film or plate is developed and employed diagnostically in medical applications and for quality control, record-keeping and scientific investigation in medical, industrial and scientific applications. Depending on the exposure radiation, the nature of the sample and the photochemistry of the photographic medium and its processing, results in a film radiograph that may possess a continuous tonal gradation in transmissivity to light extending between fully transparent (light) and fully opaque (black). The accurate reproduction of copies of the developed image is dependent on the ability of the techniques employed to faithfully reproduce the gray level gradation between the black and white extremes in the original radiographic image. Direct copying of the image has typically been attempted by photographic and xerographic techniques which rely upon exposure of a second photographic medium or xerographic drum to light transmitted through or reflected by the original image. Losses in tonal density and balance may occur, particularly in the xerographic reproduction process. In the photographic reproduction process, it is still necessary to develop the copy.
More recently, radiographic film images have been scanned by laser scanners to develop a digitized image field of the tonal density of the original image and to store the digitized image for transmission to remote locations and/or subsequent reproduction of the image. The digitization and storage of the image field also provides a back-up to the original which may be lost, particularly if it is sent to another location to be viewed by specialists in the field of interest.
In addition, digitally captured diagnostic images are generated in the first instance by computer automated tomography (CAT), magnetic resonance imaging (MRI), ultrasound, and other tomographic and stereoscopic scanners. Such digitally captured diagnostic images are conventionally stored in magnetic tape or optical disk archives and displayed on high resolution monitors for primary diagnosis in medical applications and quality inspection, scientific investigation and the like in other applications. The archives inherently provide the back-up to safeguard against loss and reduce the need for photographic film processing equipment and storage space for the resulting film radiographs. In addition, the digitally captured diagnostic images allow for telecommunications of these images from location to location.
The Kodak Ektascan.RTM. Image Link products represent a system that provides the generation, archiving and communications of such digitally captured diagnostic images captured from magnetic resonance imaging (MRI), computer automated tomography (CAT), and other modalities, such as images acquired from film radiographs through laser scanning. In such systems, images can be stored and retrieved with advanced optical disk archiving, and manipulated to suit viewing requirements and preferences. The images can be viewed on high resolution video monitors and recorded with full fidelity on film using laser printing and reproduced on paper or a transparent media with high quality thermal printing.
In this context, it is desirable to improve the reproduction of images represented by either an original film radiograph that is digitally captured through laser scanning or original digitally captured diagnostic images or other digital images.
As is well known, conventional, binary, halftone black and white simulation of continuous tone images is accomplished by reproducing the image with black or white dots of various sizes. Depending on the convention employed, the dots may constitute pixels or subpixels arranged in cells of m lines and n columns, where each cell contains m.times.n pixels each exposed to black (opaque) or white (transparent). After printing these cells on the print medium, the eye, not being sufficiently microscopic to see the individual pixels, blends them into gray level gradations simulating the continuous tone image being reproduced. Naturally, the degree to which the size of the pixels can be reduced enhances the degree to which the halftone reproduction faithfully reproduces the continuous tone original image.
Reduction of pixel size involves increased cost and complexity of the printer systems. When assessing quality of the printer, two measures are important; the number of halftone cells per linear inch (halftone frequency) and the number of distinguishable gray steps. In print media, high quality magazines typically use 150 cells per inch (cpi). The needed number of gray steps depends on the eye's ability to distinguish closely spaced grays. For good quality print media, it has been found that about 100 gray steps are necessary. In a binary printer, the maximum number of gray steps per cell is m.times.n+1. Achieving both a high halftone frequency (150+cpi) and 100 or more gray steps in each cell is difficult and costly with a binary printer.
To meet such gray level requirements, it has been proposed to employ a hybrid halftone technique consisting of trinary or quarternary (3 or 4 gray level) pixels per cell in the paper "Hybrid (Gray Pixel) Halftone Printing," Journal of Imaging Technology, Vol. 15, No. 3, pp. 130-135, June, 1989, by W. Lama, et al. These authors assert that a trinary printer (black, white and one gray pixel level) can produce a vastly greater number of output gray steps for a given halftone cell size. Furthermore, in a quarternary printer (two intermediate gray pixel levels) the output steps can approach a continuum as viewed by the human eye. It is important to note that the number of gray steps alone does not determine the gray scale quality of the printer. The gray steps must also be arrayed properly along the output density scale, and the maximum step size should be reduced below the perceptible limit.
The authors of this paper and the paper entitled "Optimum Density Levels for Multilevel Halftone Printing," (P. G. Engeldrum, Journal of Imaging Science, Vol. 31, No. 5, pp. 220-222, Sept./Oct. 1987) present intermediate density level values optimized for print media.
One problem that can result with using halftoning techniques to reproduce gray levels is image artifacts, in particular, contouring. Contouring is an artifact that is the result of the observer being able to clearly distinguish the boundary between pixels that represent two adjacent (in gray level space) gray levels. Digital halftoning techniques are discrete in representing gray levels, in that a gray level is composed of a discrete number of black dots and white dots. Two adjacent gray levels are represented typically by one gray level having one more black dot than the other. Particularly at high densities, adjacent gray levels can be noticeable to the eye, resulting in contouring.
Another problem that can plague halftoning processes with photosensitive media is lack of uniformity of the photosensitive media coatings. Lack of uniformity of coating layers can result in the visibility of the variation as an image artifact.
In the context of printing reproductions of original film radiographs or digitally captured diagnostic images, it is desirable to make that reproduction on a transparency so that the image may be viewed by transmitting light through it in the conventional fashion while avoiding these problems and advantageously avoiding post-imaging film processing.